1. Field of the Invention
The present invention relates to a process for the removal of sulfur compounds from hot gases containing the same. It particularly relates to a process wherein a hot gas containing sulfur oxides is passed sequentially through two zones and contacted in each zone with an aqueous scrubbing medium containing a sulfur oxide absorbent.
2. Prior Art
Sulfur oxides, principally present as sulfur dioxide, are present in the waste gases discharged from many metal refining and chemical plants and in the flue gases from power plants generating electricity by the combustion of fossil fuels. In addition, hot sulfur containing gases may be formed in the partial combustion or gasification of sulfur containing fuels, such as coal. The control of air pollution resulting from the discharge of sulfur oxides into the atmosphere has become increasingly urgent. An additional incentive for the removal of sulfur oxides from waste gases is the recovery of sulfur values otherwise lost by discharge to the atmosphere. However, particularly with respect to the flue gases from power plants, which based on the combustion of an average coal many contain as much as 3000 p.p.m. sulfur dioxide and 30 p.p.m. sulfur trioxide by volume, the large volumes of these flue gases relative to the quantity of sulfur which they contain make removal or recovery of the sulfur compounds expensive. Also, while the possible by-products, such as elemental sulfur and sulfuric acid, that may be ultimately obtained from the recoverable sulfur values have virtually unlimited markets as basic raw materials, they sell for relatively low figures. Consequently, low-cost recovery processes are required.
Many processes have been proposed and investigated for the desulfurization of flue gases. Several dry processes have been proposed in which sulfur dioxide is removed either by chemical reaction with a solid absorbent or by adsorption on its surface followed by oxidation of the adsorbed sulfur dioxide. In one such process, shown in U.S. Pat. No. 2,718,453, finely powdered calcium carbonate is blown into the combustion gas to form calcium sulfate or calcium sulfite. In general, a reaction between a solid and gas is relatively slow and inefficient, being limited by the available surface area of the solid. Also certain of the resultant products do not readily lend themselves to regeneration of the starting material or recovery of the removed sulfur values.
In the molten carbonate process shown in U.S. Pat. Nos. 3,438,722; 3,438,727; and 3,438,728, sulfur oxide impurities are removed from a hot combustion gas by contacting it at a temperature of at least 350.degree. C. with a molten salt mixture containing alkali metal carbonates as the active absorbent. The spent absorbent is then regenerated chemically and recirculated. The adaptation of such a process to many older existing power-plant utility installations often presents certain economic disadvantages because of the requirements for modifying the boiler systems of these utility plants in order to obtain the flue gas to be treated at the required elevated temperature for the molten salt absorption rather than at its generally much lower exit temperature from the boiler.
Wet absorption processes are suitable for treating lower temperature flue gases. In a typical wet absorption process, the flue gas is washed with an aqueous alkaline solution or slurry. Thus the use of an aqueous slurry of calcium hydroxide or calcium carbonate has been investigated in several British power plants. Also, aqueous sodium sulfite or ammonia solutions have been utilized as washing fluids.
In the wet absorption process shown in U.S. Pat. No. 3,533,748, a waste gas stream containing sulfur oxides is scrubbed with an aqueous solution of a soluble alkali such as sodium carbonate or sodium hydroxide to form sulfite and sulfate in solution. The resulting solution is then evaporated to precipitate solid alkali metal sulfite and sulfate salts, which are separated from the solution and further processed.
While these wet absorption processes have some advantages, they suffer from a common drawback of producing a liquid effluent containing a large amount of water relative to the sulfur oxide absorbed, which effluent is not amenable to simple high temperature reduction and regeneration. Thus, difficulties arise where economic and efficient recovery of the dissolved absorbent and sulfur values from an aqueous solution is attempted. In many such processes, the recovery of elemental sulfur, a preferred product, is not economical.
In U.S. Pat. No. 3,305,307 there is shown a process for the manufacture of solid alkali metal sulfite with negligible formation of alkali metal sulfate. A finely dispersed concentrated aqueous solution of an alkali metal compound such as sodium or potassium carbonate, hydroxide, or bicarbonate is passed into a substantially dry gas containing an equivalent or greater amount of sulfur dioxide, the dry gas being maintained at a temperature such that solid alkali metal sulfite is formed. To obtain a pure alkali metal sulfite by such a process, an excess reactant amount of SO.sub.2 compared with the alkali metal compound is required. Also, to avoid the formation of alkali metal sulfate, the gas containing the SO.sub.2 reactant must be relatively free of sulfur trioxide and oxidation-promoting substances such as nitrogen oxides and metal oxides, the latter being found in the fly ash. In addition, a relatively low temperature of reaction is generally required, higher temperatures promoting formation of sulfate. A principal disadvantage of this type of process is that a significant amount of particulates (reacted and partially reacted absorbent) are carried out by the gas, necessitating a subsequent filtration operation.
More recently, in application Ser. No. 300,925 filed Oct. 26, 1972 now U.S. Pat. No. 3,932,587, and assigned to the assignee of the present invention, there is disclosed a closed-cycle process for removing in a single spray drying step a sulfur oxide impurity from a hot waste gas. In accordance with the process disclosed therein a hot sulfur oxide-containing waste gas at a temperature of at least 90.degree. C. is contacted with an aqueous scrubbing medium containing between 2 and 40 wt.% of an absorbent selected from the group consisting of alkali metal carbonates, bicarbonates and mixtures thereof. The absorbent is introduced in a sufficient quantity to provide a resultant reaction product comprising a flowable, dry powder mixture containing from 20 to 85 wt.% alkali metal sulfite and sulfate and from 15 to 80 wt.% of the absorbent. The dry powder product is reacted in a molten state with a carbonaceous reducing agent under reducing conditions, to convert the sulfite and sulfate to sulfide. The molten reaction product of alkali metal sulfide and carbonate is recovered and dissolved in water to form an aqueous solution. The aqueous solution then is reacted with a carbon dioxide yielding material to form hydrogen sulfide gas as a recoverable product and regenerate alkali metal bicarbonate and carbonate for recirculation in the process.